since,afer,before,back的区别及用法?
问题描述：
since,afer,before,back的区别及用法?
答
1.since conj.既然；因为；由于；自…以来；自…以后prep.自…以来；自…以后adv.后来eg.There have been many changes since the war.从战争到现在发生了许多变化.Since we live near the sea,we can often go swim...